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Oh dear, what can the matter be?

A new experiment should help to give physics the answer to the age-old question of why matter exists. Alison Goddard reports

Why does matter exist? The question has dogged physics for decades. Earlier this month, physicists began a new experiment called BaBar, which should help to resolve the issue.

"We know that 15 billion years ago, when the universe came into existence, there were equal amounts of matter and antimatter," says George Lafferty of the University of Manchester. "If you start with nothing but energy, you must create equal amounts of matter and antimatter."

In the early universe, matter and antimatter should have annihilated on contact, producing only light. The mystery, then, is why almost all of the antimatter disappeared, allowing the universe to evolve.

BaBar, which is based at the Stanford Linear Accelerator Center in California, will fire beams of electrons at their antimatter counterparts, called positrons. This process will create B mesons - particles containing a "bottom" quark - and anti-B mesons, called B and B-bar respectively. Such particles are unstable and live only for a million millionth of a second before decaying. By studying these mesons, physicists hope to gain an insight into the lack of symmetry between matter and antimatter.

"We will be able to look at B - B-bar oscillations ," says Lafferty. "We suspect that B mesons turn into B-bar mesons more often than B-bar mesons turn into B mesons. If we can observe that directly, it will give us a handle on what happened in the early universe."

Such an effect is known as CP violation. If matter were the exact opposite of antimatter, it should be possible, in theory, to change a particle's charge and parity - replacing matter with antimatter, then reflecting it in a mirror and flipping it upside down - and the particle would still behave in exactly the same way. But in some instances this does not happen.

Physicists first uncovered CP violation in the 1960s, in kaons - particles containing a "strange" quark. They found that kaons turn into antikaons slightly less readily than antikaons turn into kaons. But kaons are relatively simple particles and to better understand the asymmetry between matter and antimatter, physicists need to study the heavier B mesons.

"We have a theory but it has only been seen in kaon systems," says Lafferty. "To really get to grips with what is going on, we need to study B mesons."

The first results from BaBar are expected in a year's time. But the experiment is in competition with a Japanese version, called Belle, which is due to start up this summer.

A further experiment is planned for the Large Hadron Collider at the Cern particle physics laboratory in Geneva, which is due for completion in 2005.

Physicists hope that the experiments will reveal chinks in the standard model, which represents particle physicists' best attempts to understand the universe, but which cannot yet account for the missing antimatter.

"There are two possible consequences," says Lafferty. "The first is that the level of CP violation agrees with the standard model. That would be a good thing, since it would verify the standard model, but it would still leave us with the mystery of what happened to the antimatter. The other is that the level disagrees, which would mean that everything is up for grabs.